1. Field of the Invention
The invention is based on a high-temperature superconductor according to the preamble of patent claim 1. The invention also relates to a use of the high-temperature superconductor.
2. Discussion of Background
In the preamble of patent claim 1, the invention makes reference to a prior art as is disclosed in DE-A1-4 124 980. In the case of the hollow-cylindrical, stablilized, ceramic, high-temperature superconductor specified there, noble metal bars or conductors are provided within said high-temperature superconductor in good electrical contact with it and parallel to its longitudinal axis. The hollow cylinder is provided at its ends with two contact pieces which are made of sheet silver and are connected to one another via the electrical noblemetal conductors.
U.S. Pat. No. 5,140,290 discloses a device for inductive current limiting of an alternating current, in the case of which device the current which is to be limited flows through an induction coil. Arranged in the interior of this coil is a hollow cylinder, made of a high-temperature superconductor, and a soft-magnetic material of high permeability is arranged concentrically therein. During normal operation and at the rated current, the superconductivity of the hollow cylinder shields its interior, so that the impedance of the induction coil is very low. In the case of an overcurrent, for example as a result of a mains short, the superconductivity disappears and the impedance of the induction coil reaches its maximum, current-limiting value.
These voltage and current loads in the event of brief overcurrents above the critical current and with electrical voltages of several mV/cm to V/cm lead to so-called hot spots. Local increases in the electrical voltage occur as a result of small inhomogeneities in the material of the high-temperature superconductor. These lead to increased energy dissipation and thus to heating at this point. The consequence is an increasing local elevation of the resistance and hence of the voltage drop. In the event of a relatively long load, this effect leads to local destruction of the high-temperature superconductor.
DE-A1-4 019 368 discloses a method for producing cylinders or rings of a high-temperature superconductor based on a bismuth 2-layer cuprate. In this case, the homogeneous melt is cast into a rotating, cold casting mold. As a result of highly different solidification rates at the edge and in the interior of the sample, a structure is produced having a highly different density and being full of internal stresses.
The sensitivity of such high-temperature superconductors, as are used, for example, for shielding electromagnetic fields at temperatures below 100K or as inductive current limiters, is due to the lack of any plastic deformation capability of the ceramic. In the event of tensile loading, stress peaks are produced at very small microscopic cracks, which stress peaks cannot be reduced plastically and lead to the cracks growing. The mechanical tensile stresses are caused by, for example, electromagnetic forces or temperature gradients.
In terms of the relevant prior art, reference is additionally made to DE-A-1 765 109, which discloses a stablilized alternating-current superconductor, in the case of which a conventional Type III superconductor, made of, for example, technetium or niobium/zirconium with a layer thereon of Type I or II superconductor of lead or niobium with a thickness of 1 .mu.m-10 .mu.m in each case is arranged, for example by dissociation, electroylsis or plasma deposition, in good electrical contact, on a hollow-cylindrical carrier made of copper or aluminum.
For ceramic high-temperature superconductors, specially dimensioned metal layers are required for electrical stabilization. In this case, making electrical contact is problematic.
DE-A1-3 919 465 discloses a current-limiting inductor coil having a winding through which current passes and having a hollow-cylindrical, superconductive core made of a metal-oxide ceramic superconductor which changes magnetically into a normally conductive state if a current threshold is exceeded in the winding. The cavity in the superconductive core is at least partially filled with a ferromagnetic material which can be alternately composed of superconductive and ferromagnetic material and can be thermally insulated from the superconductive core.
Once again, local hot spots can occur in the even of overcurrents, as a result of small inhomogeneties in the superconductor.